1. Field of the Invention
The present invention relates to a layer forming method and apparatus suitable for use in the fabrication of various electronic devices of the type having an insulating, protecting, conductive, semiconductor or like layer formed on a substrate member.
2. Description of the Prior Art
Heretofore there has proposed a method for forming such a layer on a substrate member through use of a photo CVD process.
According to the method utilizing the photo CVD technique, the substrate member is placed in a reaction chamber provided with a light transparent window and a reactive gas mixture, which contains at least a gas of a material for the formation of the layer desired to obtain, is introduced into the reaction chamber. Then light is introduced into the reaction chamber through the light transparent window thereof by which the reactive gas mixture introduced thereinto is excited for vapor-phase decomposition and the material for the layer is deposited on the substrate member.
With the photo CVD process, since the material gas resulting from the vapor-phase decomposition of the photo-excited reactive gas is not accelerated, it is possible to form the layer on the substrate with substantially no damage inflicted on the substrate surface. On this account the layer can easily be formed without containing the material forming the substrate surface or without introducing into the substrate surface the material forming the layer, without developing any undesirable interface level between the layer and the substrate and without applying any internal stress to the layer and the substrate. Furthermore, since the photo-excited material gas has a characteristic to spread on the surface of the substrate member, the layer can be deposited in close contact with the substrate even if the substrate surface is uneven.
Accordingly, the use of the photo CVD technique permits easy formation of the layer of desired characteristics, without causing any damages to the substrate surface, even if the substrate has an uneven surface.
With the photo CVD process, however, an unnecessary layer of the material for the layer is deposited as well on the light transparent window during the formation of the layer. On this account, in the case where after the formation of the layer on the substrate member, a new layer of a composition identical with or different from that of the abovesaid material is formed on a new substrate member in the reaction chamber, the light to be introduced into the chamber through the light transparent window is partly intercepted by the unnecessary layer deposited on the window, so that a reactive gas mixture cannot effectively excited. Consequently, much time is needed for depositing the new layer. A conventional solution to this problem is to coat the window with oil of fluorine series which prevents the deposition of such unnecessary layer. But, when the window is given such a coating of oil, the layer being formed on the substrate is doped with an unnecessary impurity contained in the coating.